The present invention relates generally to charging devices and in particular to charging devices which produce a negative corona.
In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member may be charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle.
Various types of charging devices have been used to charge or precharge photoconductive insulating layers. In commercial use, for example, are various types of corona generating devices to which a high voltage of 5,000 to 8,000 volts may be applied to the corotron device thereby producing a corona spray which imparts electrostatic charge to the surface of the photoreceptor. One particular device would take the form of a single corona wire strung between insulating end blocks mounted on either end of a channel or shield. Another device, which is frequently used to provide more uniform charging and to prevent overcharging, is a scorotron which comprises two or more corona wires with a control grid or screen of parallel wires or apertures in a plate positioned between the corona wires and the photoconductor. A potential is applied to the control grid of the same polarity as the corona potential but with a much lower voltage, usually several hundred volts, which supresses the electric field between the charge plate and the corona wires and markedly reduces the ion current flow to the photorecepetor.
A recently developed corona charging device is described in U.S. Pat. No. 4,086,650 to Davis et al., commonly referred to in the art as a dicorotron wherein the corona discharge electrode is coated with a relatively thick dielectric material such as glass so as to substantially prevent the flow of conduction current therethrough. The delivery of charge to the photoconductive surface is accomplished by means of a displacement current or capacitive coupling through the dielectric material. The flow of charge to the surface to be charged is regulated by means of a DC bias applied to the corona shield. In operation an AC potential of from about 5,000 to 7,000 volts at a frequency of about 4 KHz produces a true corona current, an ion current of 1 to 2 milliamps. This device has the advantage of providing a uniform negative charge to the photoreceptor. In addition, it is a relatively low maintenance charging device in that it is the least sensitive of the charging devices to contamination by dirt and therefore does not have to be repeatedly cleaned.
In the dicorotron device described above the dielectric coated corona discharge electrode is a coated wire supported between insulating end blocks and the device has a conductive auxiliary DC electrode positioned opposite to the imaging surface on which the charge is to be placed. In the conventional corona discharge device, the conductive corona electrode is also in the form of an elongated wire connected to a corona generating power supply and supported by end blocks with the wire being partially surrounded by a conductive shield which is usually electrically grounded. The surface to be charged is spaced from the wire on the side opposite the shield and is mounted on a conductive substrate.
In addition to the desirability to negatively charge one type of photoreceptor it often is desired to provide a negative precharge to another type photoreceptor such as a selenium alloy prior to its being actually positively charged. A negative precharging is used to neutralize the positive charge remaining on the photoreceptor after transfer of the developed toner image to the copy sheet and cleaning to prepare the photoreceptor for the next copying cycle. Typically in such a precharge corotron an AC potential of between 4,500 and 6,000 volts rms at 400 to 600 Hz may be applied. A typical conventional corona discharge device of this type is shown generally in U.S. Pat. No. 2,836,725 in which a conductive corona electrode in the form of an elongated wire is connected to a corona generating AC voltage.
In the above referenced, copending applications, certain difficulties were observed when using corona charge devices that produced a negative corona. It is believed that various nitrogen oxide species are produced by the corona and that these nitrogen oxide species are adsorbed by solid surfaces. In particular it is believed that these oxide species are adsorbed by the conductive shield as well as the housing of the corona generating device. The shield may in principle be made from any conductor but is typically made from aluminum and the housing may be made from any of a number of structural plastics such as a glass filled polycarbonate. This adsorption of nitrogen oxide species occurs despite the fact that during operation the corona generating device may be provided with a directed air flow to remove the nitrogen oxide species as well as to remove ozone. In fact during the process of collecting ozone the air flow may direct the nitrogen oxide species to an affected area of the charging device or even some other machine part. It has also been found that after such exposure when a machine is turned off for extended periods of idleness that the adsorbed nitrogen oxide species gradually are desorbed, that is the adsorption is a physically reversible process. It should be understood that the adsorbed and desorbed species are both nitrogenous but not necessarily the same, i.e., there may be conversion of NO.sub.2 to HNO.sub.3. Then, when the operation of the machine is resumed, a copy quality defect is observed in the copies produced in that a line image deletion or lower density image is formed across the width of the photoreceptor at that portion of its surface which was at rest opposite the corona generating device during the period of idleness. While the mechanism of the interaction of the desorbed nitrogen oxide species and the photoreceptor layers is not fully understood, it is believed that they in some way interact with the surface of the photoreceptor increasing the lateral conductivity so that it cannot retain a charge in image fashion to be subsequently developed with toner. This basically causes narrow line images to blur or to wash out and not be developed as a toner image. This defect has been observed with conventional selenium photoreceptors which generally comprise a conductive drum substrate having a thin layer of selenium or alloy thereof vacuum deposited on its surface as the imaging surface. The difficulty is also perceived in photoreceptor configuration of plates, flexible belts, and the like, which may include one or more photoconductive layers in the supporting substrate. The supporting substrate may be conductive or may be coated with a conductive layer over which photoconductive layers may be coated. Alternatively, the multilayered electroconductive imaging photoreceptor may comprise at least two electrically operative layers, a photogenerating layer or a charge generating layer and a charge transport layer which are typically applied to the conductive layer. For further details of such a layer attention is directed to U.S. Pat. No. 4,265,990. In all these varying structures several of the layers may be applied with a vacuum deposition technique for very thin layers.
Furthermore with prolonged exposure of the photoreceptor to the desorbing nitrogen oxide species during extended periods of idleness the severity of the line defect or line spreading increases. While the mechanism is not fully understood it has been observed that even after a relatively short period of time, 15 minutes, and a period of idleness of, say, several hours, a mild line defect and concurrent image deletion may be perceived. During the initial stage of exposure of the photoreceptor to the desorbing nitrogen oxide species, it is possible to rejuvenate the photoreceptor by washing with alcohol since reaction between the photoreceptor and the nitrogen oxide species is purely at the surface. However after a prolonged period of time the reaction tends to penetrate the photoreceptor layer and cannot be washed off with the solvent. Thus, for example, the problem is perceived after a machine has been operated for about 10,000 copies, rested overnight and when the operator activates the machine the following morning, the line deletion defect will appear. As indicated above the defect is reversible to some degree by a rest period. However, the period involved may be of the order of several days which to an operator is objectionable.
Similar difficulties are encountered in a precharge corotron with a negative DC potential applied. Attempts to solve that problem by nickel plating the corotron shield met with limited success in that nickel combined with the nitrogen oxide species forming a nickel nitrate which is a deliquescent salt and on continued use becomes moist with water from the air eventually accumulating sufficient water that droplets may form and drop off onto the photoreceptor. Furthermore the nickel nitrate salts are green crystalline and loosely bonded rather than a cohesive durable film. In another attempt to solve a similar difficulty in a negative charging AC dicorotron device the shield is coated first with a layer of nickel that is subsequently plated with gold. However, as a result of the extreme expense of gold, the gold is plated in a very thin layer and consequently the layer is discontinuous having numerous pores in the layer. Gold plating is theorized to provide a relatively inert surface which will not adsorb the nitrogen oxide species or will not permit conversion to a damaging form. However with the thin porous layer of gold, the nickel substrate underneath the gold corrodes forming nickel nitrates in the same manner as with the precharge corotron and experiences similar difficulties resulting in limited useful life.
U.S. application Ser. No. 680,861 addresses this problem and provides a solution by means of plating the elements capable of absorbing nitrogen oxide species with a thin layer of lead. U.S. application Ser. No. 680,867 addresses the problem and teaches a remedy by providing continuous thin layer of a paint containing a reactive metal on the surfaces which absorbed the nitrogen oxide species. U.S. application Ser. No. 680,879 and U.S. application Ser. No. 703,971 addresses the problem and provide an alkali metal silicate coating on the elements capable of absorbing and neutralizing the nitrogen oxide species.
While capable of performing satisfactorily in the applications, according to the above reference copending applications, certain difficulties were observed with regard to use of alkali metal silicates as a coating on the conductive control grid in the above referenced type of scorotron charging device. In particular, it was observed that after about 8 hours use a white powder, presumably an alkali metal nitrate collected on the grid. The white powder on the grid was found to alter the electrostatic relationship in the charging devices in that the current delivered to the control grid from the coronodes and the current delivered to the photoreceptor began to vary uncontrollably thereby providing an unpredictable, uneven charge on the photoreceptor, resulting in poor copy quality. The exact mechanism by which this happens is not fully understood, but is believed to be a combination of the holes becoming clogged with the white powder, the resistive nature of the coating, and the particulate nature of the nitrate powder. In particular, the ratio of the current to the control grid to the photoreceptor is determined generally by the geometry of the control grid, so if the holes are plugged, that geometry and the ratio of the current to the grid to the photoreceptor is altered. The resistive nature of the nitrate powder causes it to change the effective bias on the grid by an amount equal to the voltage drop across the resistive powder layer. An finally, the particulate nature is believed to cause non-uniform electrical fields which in general tend to increase the current from the coronode.